Abstract

We have investigated linear and nonlinear optical properties of surface immobilized goldnanospheres (SIGNs) above a goldsurface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembledmonolayers(SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be for the SIGN system of nanospheres in diameter with a gap distance of . The corresponding susceptibility was estimated to be . In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the goldnanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.